Mirror modifier distort ambient occlusion texture on mirror axis
I think your problem is not the mirror modifier but the Subdivision Surface modifier.
It has a checkbox Subdivide UVs which is enabled by default.

If it was enabled during baking it can create this kind of black seams if you


delete the subsurf modifier
do not display the subsurf modifier
lower the subdivision level
or disable Subdivide UVs afterwards


I guess you want a low poly character with smooth AO, so you probably deleted the subsurf modifier. In this case you should just deactivate Subdivide UVs during baking.